Presently, conventional processes for manufacturing integrated circuits (“ICs”) include a number of process steps in which a surface of a semiconductor wafer is first coated with a thin layer of a photo-resist material after which the photo-resist material is irradiated with short wavelength light to form a latent image of a pattern in the photo-resist layer. A subsequent processing step develops the latent image thereby leaving patterned photo-resist material on a wafer's surface. In processing a semiconductor wafer to fabricate ICs, the preceding procedure for establishing patterned photo-resist material on a wafer's surface may be repeated dozens of times.
In irradiating photo-resist material, the short wavelength light used in forming the latent image passes first through a reticle before impinging upon the thin layer of photo-resist material. In general, reticles used in IC fabrication are made from a thick, planar, rectangularly or square shaped pieces of glass. The reticle is opaque in those areas of the pattern where the reticle blocks the short wavelength light from impinging upon the thin layer of photo-resist material. The pattern formed in the photo-resist layer on a wafer's surface generally differs for each of the dozens of photo-resist exposures performed during IC fabrication. Thus, fabricating a particular type of IC may require a dozen or more reticles.
Because reticles are high precision optical devices, they are comparatively expensive. Each individual reticle can cost between $5,000.00 and $30,000.00 depending upon the size of the smallest feature in the pattern to be formed in the photo-resist layer on the wafer's surface. Consequently, a complete set of reticles needed for fabricating a single type of IC may cost several hundred thousand dollars. Correspondingly, the photolithographic equipment which receives both the reticle and the wafer for exposing the photo-resist layer to short wavelength light is also comparatively expensive costing several million dollars.
A typical IC factory, commonly referred to as a “fab,” may include several different models of photolithographic equipment from different manufacturers, or different models of photolithographic equipment from the same manufacturer. While these differing models of photolithographic equipment will all accept the same set of reticles used in manufacturing a single type of IC, previously there has existed no standard cassette for holding the set of reticles while individual reticles are automatically loaded into and removed from the photolithographic equipment. That is, individual photolithographic equipment manufacturers have arbitrarily selected unique configurations for cassettes used for holding reticles while individual reticles are automatically loaded into and removed from the photolithographic equipment. Thus, worldwide presently there are in daily use in IC fabs reticle cassettes having dozens of different, incompatible configurations. Consequently, if a set of reticles for manufacturing a particularly type of IC are loaded into a cassette for a particular type of photolithographic equipment and that particular photolithographic equipment is unavailable while another model of photolithographic equipment is available, presently the reticles must be manually moved from one style of cassette that is incompatible with the available photolithographic equipment to another style of cassette that is compatible with the available photolithographic equipment.
In an effort to standardize reticle cassettes among the products of various photolithographic equipment manufacturers, recently Semiconductor Equipment and Materials International (“SEMI”) has adopted a standard, i.e. SEMI E100-0302, entitled “Specification for a Reticle SMIF Pod (RSP) Used to Transport and Store 6 Inch or 230 mm Reticles.” As implied by the name of the SEMI standard, the configuration of RSP is an adaptation of a previously existing Standard Mechanical InterFace (“SMIF”) pod which is widely used in IC fabs for carrying 8-inch semiconductor wafers during wafer processing. While it appears likely that sometime in the future all photolithographic equipment will accept the RSP for holding a set of reticles while individual reticles are automatically loaded into and removed from the photolithographic equipment, due to the presently existing large installed base of photolithographic equipment such a situation is unlikely to occur in the immediately foreseeable future. Thus, for the foreseeable future a need will continue to exist for automatically moving reticles from one style of cassette to another style of cassette.
Other considerations existing in conventional photolithographic processing exacerbate the need to move reticles from one style of cassette to another style of cassette. The physical properties of the thin photo-resist layer degrade over time. While the severity of such degradation increases with the interval that elapses between coating a wafer's surface and exposing the photo-resist material, depending upon precise characteristics of environmental conditions to which the photo-resist layer is exposed, the interval after which the yield of good ICs from a wafer becomes economically unacceptable can be as short as tens of minutes up to several hours. Thus, once a wafer has been coated with a thin layer of photo-resist material there exists a significant economic incentive to expose the photo-resist layer as quickly as practicable. Consequently, if reticles for a particular type of IC are held in a cassette for a model of unavailable photolithographic equipment, there exists a correspondingly significant economic incentive to quickly move the reticles from one style of cassette to another style of cassette that is compatible with an available model of photolithographic equipment. However, a lack of commonality among the dozens of different reticle cassette configurations has prevented the development of automatic equipment for transferring reticles between cassettes having differing configurations.
As is well known to those skilled in the art of IC fabrication, contamination must be reduced as much as practicable, or even eliminated if possible, within an IC fab. Consequently any automatic equipment for transferring reticles between cassettes having differing configurations must preserve the cleanliness of the fab, particularly cleanliness of reticles passing through the equipment.